1. Field of the Invention
The present invention relates generally to the field of low noise amplifiers (LNAs) used in tuners. More specifically, the present invention relates to LNAs used in the tuners of set-top boxes, cable modems, and high definition television (HDTV) devices. Tuners of this type can also be used in personal video recorders (PVRs), out-of-band (OOB) devices, analog TV, and voice over internet protocol (VOIP) devices, as well as numerous other applications.
2. Related Art
Due to the complexity and availability of entertainment program source material, multiple television tuners are included in most set-top cable boxes and cable modems. These additional features, material, and functionality cause the set-top boxes to be significantly more complex and include increased functionality. Examples of this increased functionality include the ability to watch simultaneous channels, such as the popular picture-in-picture (PIP) television viewing mode and/or the ability to watch one channel while simultaneously recording another channel using a video cassette or a personal video recorder (PVR). In traditional cable-set boxes, passive splitters with sufficient bandwidth are used to split the incoming cable signal into multiple outputs in order to drive multiple tuners to provide PIP or facilitate the viewing of one channel and the simultaneous recording of another. Unfortunately, however, the use of the passive splitters degrades the overall performance of the tuner.
For example, passive splitters are known to create insertion losses of about 6 dB, which significantly degrades the performance of the associated tuners. This level of degradation, in turn, impacts the overall system""s noise figure and signal-to-noise ratio (SNR). An additional drawback of passive splitters in cable set-top boxes is that they do not permit the full use of cable signal ranges. Finally, passive splitters require extensive printed circuit board area and necessitate bulky metal configurations for electromagnetic shielding in order to minimize interference.
What is needed, therefore, is an alternative to the passive splitters currently used in cable television tuners or cable modem type applications. More specifically, what is needed is a tuner implementation that can improve upon the performance of passive splitters, such that splitting the input signal into multiple signals will not significantly degrade the overall performance of the tuner. This improved approach must facilitate the full use of the associated cable signal ranges. The improved approach must also reduce space requirements, reduce cost, and ease many of the manufacturing issues surrounding production of conventional tuners used in the aforementioned applications.
What is needed is a technique to provide autonomous automatic gain control (AGC) within the amplification modules associated with cable tuning and similar systems. Additionally, what is needed is an improved tuner with autonomous AGC that can be integrated onto a single integrated circuit (IC) and be implemented, for example, in complimentary metal-oxide semiconductor (CMOS) technology.
Consistent with the principles of the present invention as embodied and broadly described herein, an exemplary method comprises receiving a supply voltage signal and a differential input signal as a circuit input. A differential amplifier drive signal is produced in response to the received supply voltage signal, the received differential input signal, and the received differential control signal. The received differential input signal is adjusted to a value where magnitudes of negative and positive components of the differential control signal become equal to one another and are within a predetermined amount of a magnitude of the supply voltage signal.
The present invention also includes an exemplary method for processing a signal in an amplifier including a first amplification module, a number of second amplification modules, and a gain control device. The method comprises receiving a transmitted signal in the first amplification module and applying a first level of amplification thereto. Next, the method includes providing the first level amplified signal to each of the number of second amplification modules to produce a number of second level amplified signals. Each of the number of second level amplified signals has characteristics substantially similar to the other of the number of second level amplified signals. The method also includes detecting a signal level of each of the number of second level amplified signals and producing a gain control signal based upon the detected signal level. Finally, the method includes providing the gain control signal as an input to each of the amplifiers of the second amplification module and controlling a gain of the second amplification module based upon the provided gain control signal.
Features and advantages of the present invention include the integration of an active splitter tuner arrangement onto a single IC, having a relatively small physical size and footprint, in order to drive multiple tuners. This particular technique is also effective to improve second and third order distortion product performance by using a differential circuit topology. The technique of the present invention also provides AGC for associated amplification modules by providing the gain control function integrated with the amplification module. Finally, the present invention facilitates full use of the cable signal range with little or no noise figure penalty while also reducing space requirements, reducing costs, and easing the manufacturability cable set-top boxes.
Other features and advantages of the present invention include completely autonomous local AGC of variable gain type amplifiers which facilitates maximizing the output SNR plus distortion ratio of the associated tuners. An independent AGC function is included for each output to cover both positive and negative cable roll-off. Current steering circuits are used in second stage amplification modules to switch in or switch out gain smoothly with minimum impact on linearity. Techniques in accordance with the present invention also facilitate providing signals proportional to amplifier gains and are scalable to accommodate different numbers of multiple tuner applications without creating a significant impact on broadband bandwidth. Finally, the present invention combines high linearity and low noise in a distribution amplifier for multiple tuner applications with a large variable gain range.